The field of the present invention is ceramic vaporizer boats for vaporizing metal using such boats.
Metal-vaporizing methods and suitable vaporizer boats are generally known from the prior art. These prior art vaporizer boats are made of an electrically conductive ceramic material, with titaniumdiboride being commonly used for the conductive phase, and boron nitride or boron nitride plus aluminum nitride for the insulating phase. In the prior art, these vaporizer boats are made in the form of elongated rods with a substantially rectangular or square cross section. Typical cross-sectional dimensions are 10.times.20 mm, 10.times.30 mm or 12.times.19 mm. Typical rod lengths are, for example, 150 to 250 mm. The longitudinal ends of the vaporizer boats are connected to a power source, so that there is a certain current per unit of cross-sectional area which flows homogeneously through the vaporizer boats in a longitudinal direction, so that the entire vaporizer boat acts as a heating resistor and is, thus, relatively homogeneously heated. It is common practice to provide these vaporizer boats with front-surface contacts by pressing clamping jaws against the front surfaces of the respective vaporizer boat. The clamping jaws themselves are usually connected to a water-cooled electric conductor which is preferably made of copper. The ceramic material of the vaporizer boat may typically have a resistivity of 800 .mu..OMEGA.cm, for example.
In conventional vaporizing methods, the metal to be vaporized is usually aluminum or one of its alloys and is supplied onto the vaporizer boat in the form of a wire, for example, in order to initiate the melting process and to cause the vaporizer boat to be wetted by the melt. Since the cross-sectional area of the vaporizer boat is relatively evenly heated, which is due to the substantially homogeneous resistivity, the vaporizer boat is in its transverse direction also relatively evenly wet by the aluminum melt. However, this wetting characteristic also causes the aluminum to flow laterally over the longitudinal edges of the vaporizer boat, so that the metal vaporizes horizontally from the lateral surfaces instead of being vaporized in an upward direction. Common vaporizers are designed such that the surface to be coated by vaporization is moved past in a position above the vaporizer boat, e.g. a capacitor film for the manufacture of wound capacitors. An undesirable lateral vaporization, however, entirely or at least partially prevents the vaporized aluminum particles from reaching the capacitor film to be coated by vaporization. Instead, the metal will be deposited somewhere else, e.g. in some place on the wall of the evacuated chamber where the vaporizer boat and the area to be coated by vaporization are disposed. The result is a considerably more difficult and expensive maintenance of the vaporizer, because the chamber is gradually clogged by aluminum deposits which are very difficult to remove.
To prevent the aluminum melt from flowing over the lateral edges, prior art vaporizer boats have been provided with a cavity formed by milling, for example, and extending longitudinally along the vaporizer boat. The aluminum melt could thus spread inside the cavity like in an elongated crucible. It was found, however, that the melt would also flow over the transverse edges of the cavity, run down over the longitudinal edges of the boat and finally vaporize there. It was not practical either to provide the longitudinal recess in the vaporizer boat with any desired depth, since this produced an undesirably large cross-sectional area of the melt, which in turn unfavorably changed the conditions of the process. More specifically, an excessive amount of current would be conducted through the melt itself and would bypass the ceramic material forming the heating resistor. Apart from this, it was rather expensive to produce the cavity or trough by milling.
In addition to preventing the melt from flowing lateral over the longitudinal edges, it is also desirable to minimize the energy losses from the vaporizer boat caused by thermal radiation. At the same time, however, it has to be ensured that the shape of the vaporizer boat can still be manufactured easily and cost-effectively.
The U.S. Pat. No. 2,996,412 discloses an inhomogeneous vaporizer boat which is not included in the generic category and which is not exclusively made of ceramic material. A metallic strip of superior conductivity is placed in the center of a carbon substrate. Because the central metallic strip heats up to a higher temperature than the substrate surrounding it, the central portion is supposed to heat up to a higher temperature as well in order to localize the melt disposed along the central portion.
The printed specification Xerox Disclosure Journal, volume 1 no. 6, June 1976, US, page 19, discloses a tube having a highly reflective upper surface and a highly emissive (black) lower surface. These different reflection characteristics are to result in the temperature at the highly emissive surface being higher than that at the highly reflective surface. A localization of the melt is not achieved by means of these different reflection characteristics of the upper and lower surface, and this is also not intended since there is not even a theoretical possibility that the melt might flow over the lateral edges of the receiving cavity having the cross section of a semicircular tube.
The printed specification U.S. Pat. No. 3,117,887 very generally discloses a conventional ceramic vaporizer boat having a rectangular cross section.
The printed specification E. D. Parent "Bar Sources in Metallizing" Soc. of Vac Coaters, Proc 21st Annual Technical Conference (1978), p. 8-18, discloses conventional rectangular vaporizers which are included in the generic category and which comprise an height-to-width ratio of 1:3. Furthermore, vaporizers are known which are included in the generic category and which have a triangular cross section of undefined height-to-width ratio. This triangular cross section is intended to save energy as compared to the rectangular cross section.
The printed specification U.S. Pat. No. 4,089,643 discloses vaporizer boats having a triangular cross section in the form of an equilateral triangle, as well as in the form of isosceles triangles with an otherwise undefined height-to-width ratio. This triangular cross section is intended to save energy as compared to the rectangular cross section.